1. Field of the Invention
This invention relates generally to circuit fabrication and, more particularly, to a method for coupling a circuit component to a substrate.
2. Background of the Related Art
In the fabrication of electrical circuits, it is often desirable to couple a circuit component to a substrate. For instance, one particular example involves the coupling of an integrated circuit chip to a ceramic substrate or to a printed circuit board.
There are various methods for affixing metal contacts of an integrated circuit chip, for instance, to complementary metal contacts on a substrate. For example, a traditional soldering method could be used in which solder bumps are formed on the contacts of the substrate and flux is applied to the contacts of the integrated circuit chip. The contacts on the integrated circuit chip are then placed adjacent the solder bumps on the substrate and the entire assembly is heated to melt the solder. The flux attracts the solder to the contacts on the integrated circuit chip to ensure proper bonding.
Once the bonding has taken place, the flux residue is removed, because any flux which remains may create conductivity or corrosion problems that may adversely affect the performance of the circuit. However, the space between the circuit component and the substrate is typically quite small, usually on the order of a few thousandths of an inch. Due to this constraint, it is difficult for currently used cleaning mediums to penetrate and remove all flux residue from in and under the circuit component.
A process known as thermocompression ultrasonic gold to gold diffusion may be used to create such contacts without the use of flux. In this process, gold contacts are provided on the integrated circuit chip and on the substrate. The gold contacts of the integrated circuit chip are then placed adjacent the gold contacts of the substrate and pressure is applied onto the chip in the amount of approximately 10 psi. Ultrasonic energy of approximately 100 kH is applied to the structure, and the entire assembly is heated to a temperature in the range of 300EC. to 400EC. These conditions cause the gold atoms in the pairs of adjacent contacts to diffuse into one another to couple the contacts together.
Although this process solves the problem of flux removal, it still exhibits various disadvantages. First, the cost of the gold contacts may be an undesirable expense. Second, because the contacts are fused together, rather than soldered, it is difficult, if not impossible, to remove the chip from the substrate in the event that the part needs to be reworked. Third, the pressure applied to the chip may introduce microscopic stress fractures which may adversely affect the performance of the chip. Finally, many chips may be damaged by a temperature in the range of 300/C. to 400/C.
Another method may be performed at room temperature and without flux. This method is a diffusion method similar to the one described above. However, instead of applying a relatively high temperature, a pressure of approximately 25 to 35 psi is applied to the chip, along with ultrasonic energy, in order to diffuse the opposing gold contacts together. Except for the problems related to high temperature, this process suffers from all of the problems discussed above. In fact, due to the higher pressures required, more and larger stress fractures are likely to be created in the chip during processing.
The present invention may address one or more of the problems discussed above.